Remove feature flags and the fallback command

See merge request gitlab-org/gitlab-shell!336

Build against go1.13

See merge request gitlab-org/gitlab-shell!337

Release v10.0.0

See merge request gitlab-org/gitlab-shell!333

Remove gitlab-keys script

See merge request gitlab-org/gitlab-shell!329

Refactor "go build" path constants

See merge request gitlab-org/gitlab-shell!328

Release v9.4.2

See merge request gitlab-org/gitlab-shell!331

Repurpose bin/authorized_keys script

See merge request gitlab-org/gitlab-shell!330

It's been replaced by `gitlab-shell-authorized-keys-check` and
it's also calling the `/authorized_keys` endpoint via Ruby.

Since they have the same functionality as the new binary, better
to call that instead to make it SSOT for authorized keys requests.

Fix bin/compile in GDK

Closes gitlab-development-kit#585

See merge request gitlab-org/gitlab-shell!327

Release v9.4.0

See merge request gitlab-org/gitlab-shell!325

Use go mod

See merge request gitlab-org/gitlab-shell!323

Replace symlinks with actual binaries

Closes #206

See merge request gitlab-org/gitlab-shell!324

We had `gitlab-shell-authorized-keys-check` and
`gitlab-shell-authorized-principals-check` as symlinks to
`gitlab-shell` before.

We determine the `Command` and `CommandArgs` that we build based
on the `Name` of the `Executable`. We also use that to know which
fallback ruby executable should we fallback to. We use
`os.Executable()` to do that.

`os.Executable()` behaves differently depending on OS. It may
return the symlink or the target's name. That can result to a
buggy behavior.

The fix is to create binaries for each instead of using a symlink.
That way we don't need to rely on `os.Executable()` to get the name.
We pass the `Name` of the executable instead.

Implement AuthorizedPrincipals command

Closes #181

See merge request gitlab-org/gitlab-shell!322

Build this command when `Executable` name is
`gitlab-shell-authorized-principals-check`. Feature flag is the
same name.

Implement AuthorizedKeys command

See merge request gitlab-org/gitlab-shell!321

Build this command when `Executable` name is
`gitlab-shell-authorized-keys-check`. Feature flag is the same
name.

Support falling back to ruby version of checkers

See merge request gitlab-org/gitlab-shell!318

This struct is responsible for determining the name and
root dir of the executable.

The `RootDir` property will be used to find the config.

The `Name` property will be used to determine what `Command`
and `CommandArgs` to be built.

Other functions are still expecting for `CommandArgs` instead
of `Shell`. They should be expecting `commandargs.Shell` now
since it has been renamed.

`CommandArgs` has been renamed to `Shell`.

An interface has been added that includes `Executable()` and
`Arguments()` method. The `BaseArgs` implement this methods
and should be embeeded in each type.

Rename the ruby scripts to have `-ruby` suffix and add a symlink
for both to `./gitlab-shell`. The executable name will be used to
determine how args will be parsed.

For now, we only parse the arguments for gitlab-shell commands. If
the executable is `gitlab-shell-authorized-keys-check` or
`gitlab-shell-authorized-principals-check`, it'll always fallback
to the ruby version.

Ruby specs test the ruby script, the fallback from go to ruby and
go implementation of both (still pending).

Update rubygems version on CI for go tests

See merge request gitlab-org/gitlab-shell!320

Resolve "Update .PHONY to have accurate list of targets"

Closes #198

See merge request gitlab-org/gitlab-shell!316